RAID 0 not as fast as expected

I'm experiment with RAID, looking for an inexpensive way to boost performance. I bought 4 Seagate 7200.9 120 GB SATA drives and two SIIG dual-port SATA cards. (NB: I don't plan to run RAID 0 in production, probably RAID 10, so no need to comment on the failure rate of RAID 0.)

I used this raw serial-speed test:

time sh -c "dd if=/dev/zero of=./bigfile bs=8k count=1000000 && sync"
(unmount/remount)
time sh -c "dd if=./bigfile of=/dev/null bs=8k count=1000000 && sync"

Which showed that the RAID 0 4-disk array was almost exactly twice as fast as each disk individually. I expected 4X performance for a 4-disk RAID 0. My suspicion is that each of these budget SATA cards is bandwidth limited; they can't actually handle two disks simultaneously, and I'd need to get four separate SATA cards to get 4X performance (or a more expensive card such as the Areca someone mentioned the other day).

On the other hand, it "feels like" (using our application) the seek performance is quite a bit better, which I'd expect given my hypothesis about the SIIG cards. I don't have concrete benchmarks on seek speed.

Thanks,
Craig